Battery Absorb Time

How often should you get the batteries all the way through absorb to float state?

I've got a brand new solar battery emergency backup system that I'm toying with. I've got a 380AH AGM 12V battery bank hooked to two 170 watt solar panels with a MS TriStart MPPT 60 controller. The panel position isn't optimum and so far I've seen a maximum of 800wh a day according to the TriStar.

When the batteries get full enough to move from bulk charge to absorb (at about 14.55V) that limits the incoming power from the panels a good bit. After spending over two hours in absorb mode on both Saturday and Sunday I decided to top off the batteries using a 30A wall power battery charger. I was surprised that the charger ran for about 8 hours Sunday night topping off the batteries. The charger decides to switch to float after the 14.5V absorb current gets down to 2.5A. I don't know whether this is optimum for my battery bank. The charger used almost half a kWh topping off the batteries before it got to float.

It doesn't seem right that the batteries need 12 hours of absorb voltage to get 100% charged. With shading issues I only get a third that time of half decent sun each day. How much absorb time do I need and how often to keep my batteries healthy?

My current system is admittedly weak for recharging the batteries. It would probably take me two days or more of good sun to get from 50% charge to 80% charge. But once it hits absorb state even my panels are overkill as the current is throttled to 10A to 3A into the batteries for the next three days or so.

After pulling just 5 or 10 AH out with a small load overnight the TriStar spent 180 minutes in absorb and 78 minutes in float. I'm not sure how much more I could pull out every night and still make it back to float and 100% charge. Is it necessary to get back to 100% regularly?

I doubt there is a cut and dried rule like get your batteries to 100% every day or they will fail immediately. Everything with batteries seems to be about statistics; if you do this to batteries they'll last 10% longer; if you do some other stressful thing they'll die twice as fast. But in the end lead acid batteries always fail eventually.

I'm very new to this. Am I missing something or is this yet another ugly truth about solar power?

Comments

Yes, an ugly truth about recharging batteries, as they approach full charge, they charge slower and slower. Think of siphon and 2 large tanks of water, when the tanks are un-equal, the siphon runs fast, but as they approach the same level, the siphon slows way down. Charger voltage and battery voltage.

You want to at least get the batteries into some absorb (80% full) time daily, and fully charged a couple times a week. Do you have any other loads running that could be diverting the charge ?

If the Charger was a IOTA -IQ4 series it has a automatic 8 hr Absorb time built in. It will also repeat a full charge cycle every 7 days automatically on it's own. You can fake it out if the it has the plug in Model IQ4, by unplugging it after it's in the absorb for a couple hrs and plugging it back in. The higher voltage causes it to go to float.

ABSORPTION STAGE - This state is limited to 480 minutes (8 hours) during which the charger will operate either at Full
Current output or Constant Voltage output depending on the discharged state of the battery. During Full Current output, thecharger is providing its full current rating and will slowly increase the battery voltage to the “Absorption Stage” voltage. At the end of the 480 minutes, the charger will revert to the FLOAT STAGE.

The loads I put on this system are entirely up to me. I realize it's working backwards to figure the load from a system rather than design the system for the loads, but that's the way it is. The batteries main task in life is to act as a backup power source in conjunction with an inverter in case of a utility power failure. They are sized to run the fridge for a couple days in the event of an outage. I couldn't resist adding a solar charger and a couple panels. Now I'm trying to figure whether I can put any small load on the system 24x7 and still have the batteries nearly fully charged if and when there is a power failure.

I have a TriMetric battery monitor so I can watch the battery state and loads fairly closely. The TriStar controller pulls about 2 watts during the night so that's fairly insignificant. I have an automatic transfer switch I'd like to hook my inverter to, therefore I'd like to keep the inverter running in standby. That takes about 8 watts with no load on the inverter. Now I'm up to about a 10 watt load with the inverter and TriStar doing nothing all night. Of course that little 10 watt load adds up to 240 Whr a day. That's about a third of my maximum daily solar power budget.

If my batteries are going to spend all their time in absorb mode that is probably the limit I can load the system: a couple devices in standby. If I don't need to get the batteries up close to 100% daily I could afford to add some more loads and maybe run some networking equipment purely on solar power.

Even if I add solar panels it won't help much if the batteries need to spend most their time in absorb stage. I already have plenty of power for absorb. I'm throwing away power during absorb. The only advantage to more panels is if I could position them to capture afternoon sunlight better. My current panels output drops dramatically when they get shadowed mid afternoon. It seems like the ideal system would have a split between east facing panels and west facing panels (SE and SW anyway) in order to stretch out the solar power time and ease power into the batteries.

My system is similar. 1050 watts of panels, 1000AH battery bank, running a refrigerator and a freezer. Sunny day and it will hit absorb by noon. But I leave the load on them so I don't lose solar power due to the absorb amps decreasing. The solar array will run the reefers and do the absorb easily with good sun. I don't think it has ever failed to reach float on a sunny day. Even in December in Michigan. If it gets cloudy I can flip the grid feed to the inverter/charger back on and bring it up to float in short order.

It sounds like you want to do what a lot of us off-gridders do: find something to use the otherwise "surplus" power for during the day once the batteries are up to Absorb Voltage. This is when the panels output will drop to a small percentage of their potential because that's all the current that is needed for Absorb stage. That's when load shifting comes in to play.

I'm not sure the MS controller has a programmable AUX function, but the common arrangement is to use such to switch on a load whose needs are compatible with the extra panel potential. This can also be done with a Voltage-controlled switch, with a bit of trial-and-error.

However, your basic shortcoming in panel capacity is the reason why you're spending so much time in Absorb: the longer the Bulk cycle takes, the longer the Absorb cycle will be. If you have an Absorb End Amps setting I'd go for that: around 2-3 % of the total battery bank capacity (about 11 Amps for your 380 Amp hour battery bank).

It would help to up the panel capacity: closer to 600 Watts would be recommended for full off-grid ability here. But your 340 Watts of panels is above the 5% minimum and in my opinion suitable for recharging in a back-up power situation. At best you have about 10 Amps @ 12 VDC "extra" available through Absorb, and about double that if you get to Float. You probably run out of sun first (one of the main reasons for the 10% rule-of-thumb; get them charged and into Float before sunset).

If you put a load on 24/7 you will drain the batteries somewhat and that will require longer Bulk & Absorb time. But if you "siphon off" the "extra" capacity during Absorb you will gain that power. That would be about 100-120 Watts DC, slightly less if AC (inverter power consumption & conversion efficiency loss).

I agree that load shifting to take advantage of good sun does two thing. The first is if you can use the sun for essential loads those are AH you don't have to replace. The second of course is that your are using PV at a time when your battery doesn't need the power.

As for absorb time,, personally, if your system is fairly well balanced, you are seldom going to see much absorb time. I use the trimetric as well, and in the real world if I get into the mid to high 90% range most days I am happy. Depending on the week, I see 100% at least once a week. Everything you do off grid is a balancing act, and it is never "ideal".

Do the best you can, learn as you go and you will be fine. My first set of T-105s are still on seasonal service, after 12 years,, and they were never "properly" charged. Never enough current, but nearly always enough net AHs.

i think most batteries won't need an 8hr absorb time and i suspect you won't need any more than about 2hrs or so. holding the absorb time longer holds the voltage at a higher level while the current continues to diminish. using concorde's sunxtender as an example, they state that the absorb is accomplished when the current gets down to .5% of the battery ah rating or 1/2a per 100ah of capacity. for your 390ah capacity that equates to 1.95a or 2a if good enough for government work. at that point it should be switched over to float or the charge stopped. if you have monitored the current while absorb charging you can see when this current level was reached and in what amount of time since switching over from bulk to absorb. that time is your ideal absorb time, but most controllers are in time increments so round this up to the next nearest increment in time so as to insure a full charge. for the iota, if it does not adjust on the absorb time then you may want to disconnect it or shut it off after the determined absorb time you need.

the absorb is accomplished when the current gets down to .5% of the battery ah rating or 1/2a per 100ah of capacity. for your 390ah capacity that equates to 1.95a

I am new to the off-grid life. I have a pretty fair background in science and electronics, but not much practical knowledge or experience. I have been struggling through hundreds of pages of this forum (and the many links this forum provides) to learn about battery charging. In this one short thread I can see clearly that even super moderators don't give the same advice. At this point I am following icarus's advice in this thread:

One thing that has been on my mind is the advice about bulk charging current. Most experts recommend current should be about 8% - 13% of C/20. I think I am clear on why not to exceed 13%. I am not so clear on why it should be at least 8%.

vtmaps,
actually, i was basing my statement for sunxtender agm batteries and not an fla type battery and the op indicated he had agm batteries.

we actually say 5%-13%, but this is arbitrary. it will vary for different batteries and circumstances and does represent what goes into the battery minus any possible constant loads on at the time. most batteries come rated at a 20hr rate (5%) and as such it is also fine to charge them at such a rate too. often that is in reference to a prolonged constant charging source of which solar is not. we also list it as a minimum because going lower in % many batteries have a problem in reaching a full charge and many self discharge rates become significant at some point here too. most see up to 5hrs of fully rated sun for pvs so 10hr rates have a better chance of getting a better charge to the battery in a day and to do so in 5hrs is a 20% rate of charge. they'd still need time to absorb for an hour or 2 after that.

also, note that many batteries can take more than 13% charge rate, but there are some we know won't too, so we figure that between the exceptions and the over-maintenance some batteries would need at a higher rate than this that 13% became an arbitrary number to go by. the industry actually goes by c/8 which is 12.5%, but that 1/2% isn't going to be a battery killer and makes it easier to write as a %. if one does go higher in % one has to be sure the manufacturer says it's ok to do so for you don't want to hurt your batteries or cause yourself extra maintenance. if you go way too far you could warp the plates due to excess heat. excessive discharges could do this too.

One of my favorite nits to pick, is the concept that high charge rate being need to mix elactrolyte.

In my personal observation of the Surrette banks here -- FLAs, the substantial mixing seems to occur well past the high-rate (Bulk) portion of the charge cycle. The mixing seems to occur in the later part of Asorb, and during EQ. When there is all of that gurgling/bubbling.

It IS said that EQing, among other things, reduces Stratification. EQing is beyond the Asorb stage.

Here, high rate charge at about C/10 causes almost no bubbling/grugling, The later portion of Asorb (where the charge current has tapered off significantly, and EQing does create a lot of the bubbling/mixing.

It is my opinion that the mixing is caused by the same thing that contributes to the inefficency of FLAs -- the breaking down of the water in the electrolyte into constituant parts, and the attendant warnings about ventilation of battery boxes/rooms during Asorb and EQ to clear the liberated Hydrogen.

I use 5% for several reasons (remembering, I am in no-way a battery engineer):

5% is the recommended Equalizing current by Rolls/Surrette (and a think a couple others).

5% divided by 4-6 hours per day is roughly a 1% average rate of charge over 24 hours... That is pretty much what is recommended for floating a battery bank. And older batteries (particularly forklift/traction batteries) have higher self discharge when approaching end of life (upwards of 1-2% per day). 5% would, more or less, keep up with self discharge.

5% rate of charge will take quite a while to recharge a battery bank... More or less, batteries last longer if they do not sit at low states of charge for days/weeks/etc. at a time. So, if you substantially discharge a battery bank and use a low power solar array for recharging, it will take days to fully recharge the bank. For a weekend cabin, perhaps that is OK. For a place using power for most of the year, you may need to use a genset once or twice a week to get the battery bank fully recharged (and more in the winter).

Charging (and discharging) a battery bank at C/20 is certainly gentle and more efficient than charging at C/8 (12.5%) (Puerket Effect--higher current rates are less efficient).

For solar charge controllers, you probably are not going to be charging for 8 hours at 100% of solar array/charge controller rating. This becomes (for me) more of a practical cost effective maximum. If you get 4 hours of "full sun" per day minimum, then 4*13%=52% recharging per day. Given we suggest that people do not discharge a bank below 50% state of charge for long battery life, it is probably not necessary to add more solar panels... The system will be operating at much less than 100% output for much of the day (especially during summer). So having a >>~13% array is usually just a waste of money for a "typical" off grid power system.

Elsewhere, our host recommends for AC powered battery chargers that you can go as high as 25% for some AGM (and imho, probably OK for flooded cell as most people are not deep cycling batteries down to 20% state of charge like industrial users--so overheating a battery starting at 75% state of charge--is not probably going to happen). Also, 25% of AH capacity is about the maximum recommended surge capacity for a flooded cell battery and most people will wire their battery banks for C/8 wiring. A larger power supply/charger could overheat the wiring/blow the fuse/breaker unless the system is designed for C/4 wiring (which is pretty heavy for any substantial battery bank).

In the end, the above are starting points for rule of thumb based designs. We want to have people happy with their systems and not get frustrated that after a couple years, the performance fails to meet their needs (boil batteries dry, batteries sulfate, self discharge eats into daily power requirements, etc.).

If there is a specific set of known uses, then the rules of thumb can be bent... For example a home power system charges during the day and is used in the evening/early morning (cooking, chores, etc.).

If your needs include irrigation and/or a home office/kids using computers at home/etc... Then you may want a larger array because a substantial amount of power is diverted to the daytime load--so the battery bank gets less AH to recharge for night time needs.

One common mistake that has been a recommendation by many over the years was to spend your money on an oversized battery bank.

Turns out over-sized battery banks need over sized solar arrays and/or backup generator to be properly charged. Plus the battery bank is expensive to purchase and replace 6-10 years down the road.

I think most people have convinced themselves here that you need to measure/estimate your loads and design the battery bank to support those loads (with 1-3 days of no-sun; and 50% maximum discharge in normal use). And spend the extra money (if you have any) on a larger array and a good quality charge controller + inverter + backup genset (as needed).

Batteries are consumables and getting the right sized for your power needs is more cost effective over the long term.

Also, in the end, we try to be conservative in our recommendations. For most people their loads grow over time (lots of neat little power hungry gadgets and computers out there)... So, giving yourself 25-35+% headroom will help prevent you from having to re-engineer your system 3-4 years down the road:

The closer you design your system to your actual power usage--the more attention you have to pay to the system day to day operation (spouse, kids, visitors, etc.).

Some people really enjoy doing that--And others do not. Again, power usage is a highly personal choice. We try to help folks design a system that, cost effectively, meets their needs. And, by the way, conservation in energy usage is also a very cost effective part of the off-grid (and even on-grid) energy plan (we are all pretty much cheapskates here 8)).

I absolutely agree that proper battery size compared to load is essential. I too made the mistake of having too much battery and not enough charge capacity initially.

When I built the new house, I did the calc that a smaller battery bank (properly managed) was much cheaper over the life cycle of the batteries. If a 1/2 size/cost battery bank lasts 75% as long as a double size bank, you are still money ahead, not counting the time value of the money.

This has been a great thread for me given I just set up a battery system and recently installed the battery monitor. One question I have is about not getting all the way through the charge sequence in one go. Is it advisable to continue in the Absorb or Float stage in the next charging sequence, provided the batteries are still in the Absorb or Float Voltage range? I'm currently running of a generator while I work in the shop and am working out work flow based on battery charging to optimize fuel, work and batteries.

This has been a great thread for me given I just set up a battery system and recently installed the battery monitor. One question I have is about not getting all the way through the charge sequence in one go. Is it advisable to continue in the Absorb or Float stage in the next charging sequence, provided the batteries are still in the Absorb or Float Voltage range? I'm currently running of a generator while I work in the shop and am working out work flow based on battery charging to optimize fuel, work and batteries.

This is another reason for the 10% charge rate rule-of-thumb: to have enough power available to complete the Absorb cycle and spend at least some time in Float on a good day. And yes the current potential is what knocks the sulphur off the plates and the higher Voltage of Absorb is what stirs it back in to the electrolyte solution. You might say that the 10% peak potential charge current calculation is a short-cut which usually brings all the other necessary numbers in line.

Your Absorb stage will not "continue" in the next charge sequence, it will start over. The desired sequence is: usage, Bulk charge, Absorb charge, Float charge. When the panel output or load demands make Float no longer possible you're back at usage. Sometimes the Absorb stage may be interrupted and then resume, depending on conditions (like clouds or turning on a heavy load). Also, the particular charge controller's charge parameters can make a difference. Some will "rebulk" and some won't. Some can have "Absorb end Amps" and others won't.

It gets to a point where the science of solar electric power becomes the art of solar electric power, with a need for fine-tuning each individual system according to its location and usage pattern. For the most part we have to deal with generalizations here, at least until enough detail has been determined that more specific advice is possible. That usually means the system is up and running.

As a follow up: yesterday I started the day with the batteries at 91% full according to the TriMetric, down by 32ah from a 2.1 amp load left running over night. At 12:15 the TriStar went in to Absorb state as the battery voltage hit 14.58 volts. The batteries stayed in absorb till 2:50 when the panels got shadowed. The current went from 14.5 amps to 5.7 amps during that time. Since that 2.1 amp load was running the entire time I was really only putting 3.6 amps into my 380 AH 12V battery bank at the end. The TriStar never switched to float, but the extra 2.1 amp load may have fooled it.

I pulled in 47.4 AH for the day vs. previous best figures of 61AH when it didn't hit absorb. I've tried including an image of two graphs of current for a best day and yesterday's absorb time.

Because of the way my produced power peaks and abruptly cuts off due to shadowing I'm really feeling the need for a more west facing panel without shading issues to stretch out the absorb time. It wouldn't need to be as large as the existing set, just a near match in voltage.

And I was grabbing the number we usually use for off-grid systems where you're going to be keeping things running (i.e. inverter load at least) while charging, and that will vary with a particular system. Keep forgetting to include all the little details like that which we tend to take for granted!

One of the other reasons for trying for higher charge rates is related to this: the net charge rate will be the gross rate less any power going out for loads. So you may appear to be charging at 10% but the loads are using some resulting in a actual charge rate of, say, 7%. It also helps push the sulphur off the plates and get it back into the electrolyte mix.

Hi all,
Just wanting some clarity on Absorb times.
My system is mainly used as a weekend cabin supply and the use when occupied is generally only ~2kW/day.
I have seen that it is recommended that the Absorb charge be terminated when the charge rate drops below 1% of the battery capacity which for my system will be 6 Amps.
My question is when the cabin is not occupied the system will currently only Absorb for ~30 to 40 minutes per day before going into Float for ~7 to 8 hours, is this OK?
regards,
Andrew.

Not a problem. Your FM60 adjusts Absorb time according to how long Bulk charging takes. When the batteries are used little, the Bulk time & Absorb time will be quite short. Having the batteries spend most of their time in Float will not hurt them, providing the parameters are set properly (some set the Voltage points lower for long-term non-use as it reduces water loss on flooded cells). Your weekend usage should be enough cycling to keep them "happy", as deep cycles do like to see some discharge/recharge cycling no matter what.

Battery maintenance is part science and part art. There are so many variables, and the most likely negative outcome of getting something wrong is reduced service life. Unless something is really screwed up you're looking at minor imperfections taking a year off a possible 5+ year span. If something is very wrong you will notice because the batteries will fail very quickly, like within one year or even less time.

"the Bulk time & Absorb time will be quite short"
......."providing the parameters are set properly"

Thanks for the information.

When set up by the installer the Absorb time was set at 2 hours and irrespective of the use it always went for 2 hours.
I added the absorb end amps setting and now when not occupied the Absorb time is 30-40mins.

If your loads on the inverter are low at the point where you would exit Asorb, using End Amps, that is what I would use to terminate Asorb, because using time to determine end of Asorb does not work well with a variable Depth Of Discharge.

Using Bulk time to determine Asorb time (here) works poorly when the average RATE of discharge varies -- if the batteries are discharged slowly (at a fairly constant rate, lets say), then the Bulk time is considerably shorter than the Asorb time would want to be.

You have Gel batts, so it is a bit more difficult for you to determine SOC to help set End AMps to the correct value. Others council that watching the charge current into the batteries taper to a point where the rate of change becomes small is a fairly good indication of 100% full.

As Coot and others have stated, tuning a battery based power system becomes an art when trying to get one's own system just right. Thankfully, we need not be perfect, altho sealed batts, especially Gels, are sensitive to overcharging, and do not allow direct measurment of SOC. Vic

When set up by the installer the Absorb time was set at 2 hours and irrespective of the use it always went for 2 hours.
I added the absorb end amps setting and now when not occupied the Absorb time is 30-40mins.

What parameters are you referring to - just the voltage settings?

Cheers, Andrew.

The Absorb time setting is a maximum: it will switch to Float after 2 hours regardless of whether or not the clock has "run out". This can be decreased to 1 hour or increased up to 4 hours (I think the FM's go even longer). If it was always going for the max then you don't have enough panel for the batteries and Bulk charging is taking too long.

If I read your sig data correctly, your battery bank is 24 Volts and 620 Amp hours. For that you would calculate:
31 Amps (5% charge rate minimum) * 24 Volts (minimum battery Voltage) = 744 Watts less 77% efficiency derating = 966 Watt array. You've got 760. The best you could hope for from that is a peak current around 24 Amps which is just under 4% and not really sufficient to keep the batteries up.

Although for "weekend only use" you can use the 5% minimum rate, don't fall into the old trap that a large battery bank can be recharged by a small amount of panel over enough time. This is "old school" and has been proven to be detrimental to battery life. The fact that you are not getting through Absorb in 2 hours shows this.

You've set the Absorb end Amps to what? Normally this number would be around 2 or 3 percent of capacity, or about 12 to 18 Amps for your bank.

I think if you could add just two more of those 190 Watt panels you will be happier with the system's performance.

Thanks Vic & Coot,
I set the Absorb end amps to be 6 amps (~1% of the C20 battery amps).
Looking back through the FM60's log for the last 128 days shows that the peak panel output has at times reached just over 1kW and that the peak amps out of the FM60 has been over 38 amps.
All the best, Andrew.

I guess I am on the other end of the spectrum with Absorb times, I had my system set for 2 hours from the start, once winter started (2010) I realized this was not enough time to get the batteries up so I increased it from in one hour increments and ended up with a settting of 5 hours. This has worked well throughout the past two winters, I'll adjust it now since the days are longer to 2-3 hours.

I have the "old school" system with too many batteries for my panels, 12 L-16 trojan RE, about 1900 watts of panels and the fm 80 charge controller.
I add water to the batteries about once a month in the summer and about every three months in the winter. My ending amps are about 5-10 with the 5 hour setting, battery's stay almost 90% all the time and never get below 70%. I only have to equalize a few times a year or less.
I am seeing about 5-10 amps at the end of the 5 hours when I am here, probably less when I am gone, I am here about 3 days a week.

I am at 3,300 feet and temps are cool until June-Aug and we get about 350 days of sun a year. Not sure how the longer absorb times will affect my battery life but the slow long trickle charge has worked well for keeping the SG up through the winter. My system has the temp compensation, a mate and seems to be very trouble free after almost two years of service I am very happy.

Hi Derik,
Thanks for letting us know how you have changed your settings.
I feel I will have to disable the Absorb end amps as I am concerned that issues around cloud cover etc. may result in the amps going into the batteries dropping for reasons other than their charge level (ie lack of charge from the solar panels).
I was concerned about the effects of too longer Absorb time on the battery health when the cabin was not occupied hence using the end amps setting.
Has anyone got a view about setting long Absorb times when they may not be needed?

I don't think the end amps setting is a problem for cloud cover as if the cloud covers the panell the voltage will drop below the setting you have set the absorb to.
To find the end amps you need to watch the amps with the timer set to max time, the voltage will stay the same but the amps will slowly drop to a spot where they will hit a flat spot and not drop any more . This needs to be at this flat spot for 15 minutes or so.
Using end amps is a good idea for the intermittent holiday type home as while away and every thing is turned off the absorb time will be adjusted by the end amps and will most likely be short.
While away my fm80 will typically report 15 or so minutes absorb time if there is sufficient sun. If very cloudy the absorb will go out to the 3 hours I have the timer set to.
( my end amps is set to 6 amps ).

Many thanks petertearai,
You have made me feel somewhat better about the settings on our system. I may extend my Absorb max time out as well.
EDIT : Looked at some photos of some of the status screens and see that even after 2 hours of Absorb the charge going into the batteries when in Float was >10amps, is this normal?Attachment not found.
Summary at time of above:Attachment not found.

One of the outback techs advised me to increase my absorb time to keep my batteries fully charged or close to with winter sun. I did it almost two years ago and it's worked great for me. I never adjusted any of the factory settings (amps) just the absorb time for 3 hours in the summer and 5 in the winter. The other plus to the longer absorb times is that you don't have to EQ as often. My volts are set at 29.6 and are temp compensated.

I don't have a generator so that's what's worked best for my system and batteries and the SG seems to almost always be good with this method.

When I first got my system up and running it somehow went to a 1 hour absorb time, this didn't work at all with my system and the batteries went down fast, since adjusting it I have had no issues.

Not sure about the life of the L-16 RE with the long absorb but I am sure it's better for them and undercharging them.